The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to functional imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The concept of acquiring NMR imaging data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C. 10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging (EPI) are well-known, and other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735, which are herein incorporated by reference.
Functional magnetic resonance imaging (fMRI) technology provides an approach to study neuronal activity. In conventional fMRI, mapping the eloquent cortex relies on blood oxygen level dependent (BOLD) contrast. The physiological basis of BOLD signal is the regional vasoactive response induced by neuronal activity, causing increases in regional cerebral blood flow, blood oxygen concentration, and consequently, measured MR signal. As described in U.S. Pat. No. 5,603,322, which is herein incorporated by reference, an MRI system is used to acquire signals from the brain over a period of time. As the brain performs a task, these signals are modulated synchronously with task performance to reveal which regions of the brain are involved in performing the task. Much research has been done to find tasks which can be performed by patients, and which reveal in an fMRI image acquired at the same time, regions in the brain that function in response to the tasks.
Additionally, conventional fMRI has been used extensively to study normal brain function, psychiatric conditions, learning disabilities, neurodegenerative conditions, recovery from stroke, and the relationship of eloquent cortex to brain tumors and arteriovenous malformations (AVMs). The pre-operative use of fMRI to identify eloquent cortex near resectable lesions is becoming a common clinical imaging scenario for surgical planning. Given the intricate nature of neurosurgical procedures, surgical planning that accurately takes into account the physical locations of neuronal activity during tasks important to quality of life, such as motor control, speech, vision, and hearing is highly desirable. While conventional fMRI techniques present the clinician with valuable information regarding regions of neuronal activity, fMRI is still limited in its capability to provide highly accurate spatial resolution due to the indirect nature of measuring neural activity through the BOLD effect. Furthermore, the BOLD effect in response to a functional task exhibits a large temporal delay on the order of several seconds and additional spatial misregistration between the physical location of the firing neurons and the observed hemodynamic changes can exist. As a result, a more direct means for detecting neuronal activity is desirable.
Neuronal currents produce transient and oscillatory magnetic fields at frequencies (<100 Hz) that can be directly detected by MEG. The source of these fields are thought to be small current dipoles composed of synchronous intracellular postsynaptic currents of parallel pyramidal cells in the cortical mantle and a diffuse current return path. Previous work attempting to detect these magnetic fields with MR has focused on local phase shifts and dephasing effects in T2 or T2* weighted images, as discussed in Bandettini et al., Applied Magnetic Resonance, 29(1):65-88, 2005. One potential problem with this method is the detection of bipolar, or “zero mean” temporal waveforms often seen in MEG. Unless the bipolar current pulse is carefully timed around a refocusing pulse in a spin echo experiment, the phase shift induced by the positive and negative episodes of the neuronal current will cancel each other. Also, the dephasing of the MR signal is expected to be very small due to the small size of the neuronal currents and there is considerable debate as to whether the effect can even be observed. And finally, the BOLD hemodynamic signal itself produces dephasing of the MR signal, as do many other phenomena, rendering it difficult to disentangle the origins of observed dephasing.
Recently ultra-low field MRI has been proposed to allow resonant interaction between the neuronal currents and spin magnetization. Here the B0 field is lowered to produce a Larmor frequency of less than 100 Hz and the transient neuronal magnetic fields act as resonant “excitation” pulses providing the initial tip of the proton magnetization. In this case a bipolar or oscillatory “zero mean” neuronal current is actually beneficial as long as its spectrum contains power at the Larmor frequency, which is less than 100 Hz. The sensitivity of the MR system for detecting neuronal currents at such a low B0 field is very low and as a result more sensitive methods for detecting neuronal currents are desired.